The present invention is related to a system, method and apparatus for connecting a low-voltage high-current DC source (PL) in series with a high voltage low-current DC source (PD) when additional power is required by the load to operate at maximum power (Pd,max). The invention enables the series connection to be made under full power by using the second source PL to commutate the load current and allow the first source PD to be reconfigured from series to parallel operation, doubling the current rating of the first source PD.
In a preferred embodiment, the invention doubles the current rating of the low-current high-voltage source PD by separating it into two series sources and reconfiguring the sources to operate in parallel. The output voltage of PD in parallel mode is only half its series value but this is compensated for by the addition of the series connected high-current low-voltage source PL. One feature of this invention is that it enables the second source PL to be connected in series with the first source PD seamlessly while under full power and without appreciably dropping the flow of power to the load or raising the voltage of the load. Further, this transition to a series connection can be done while operating within the rating constraints of the first and second sources PD, PL such that the power transfer is transparent to the load. As such, no anomalies in power (current or voltage interruptions, spikes, oscillations, drop-off, perturbations, etc.) are sensed by the load. Anomalies are avoided by using the second source PL to naturally commutate the load current. The resulting circuit topology allows the load current to be increased to its limit with both sources contributing power at their rated limits and the load power to be increased to its maximum, Pd,max, where Pd,max=PD+PL. A further feature of this invention is the control strategy used to control the source PD and effect the transitions between modes by means of switches (S1, S2, S3) connected to the outputs of rectifiers 1 and 2.
FIG. 1 illustrates a conventional diesel operation system 100. In system 100, an electrical load 105 is supplied by a diesel engine 110 driving a three phase, single-winding alternator 120 connected to a diode rectifier 130. The alternator/rectifier combination 120 and 130 is constrained by current and voltage ratings based on the rated engine power at full rpm. In this case, the load is generally operated at constant DC voltage with power varying in proportion to the DC input current. In some cases, the DC voltage may be reduced when the load requires lower voltage. In addition, the alternator may consist of two windings with two rectifiers and means to effect series or parallel connections of these rectifiers in a manner known to a person familiar with the technology, allowing the alternator and rectifier to be smaller for a given load. This technique is not suitable for the problem at hand.
It is desirable to operate the load at higher power by connecting a second high-current power source in series with the output of the rectifier. The second source, which is constrained at about half the rated load voltage, must be connected in series while the load is operating at full diesel power and, furthermore, must not appreciably raise the load voltage.
The problem, therefore, is to find an economically viable circuit topology and control strategy to connect a low-voltage high-current power source PL in series with a high-voltage low-current power source PD that is operating an electrical load or loads. The circuit must double the current rating of PD, not increase the load voltage, and allow for smooth connection of the second source PL at times when increased load current is required for higher power operation.
FIG. 2 illustrates a conventional parallel line connection system 200. In system 200, a diesel engine 210 drives an alternator 220 connected to a diode rectifier 230. Additional current is available to operate a load 205 at higher power if an external DC source 215 is connected in parallel with the output of the rectifier 230. When the voltage of external source 215 is greater than the output of rectifier 230, the rectifier diodes become reverse biased, the load current transfers from alternator 220 to external source 215 and, as a result, the alternator/rectifier current decreases to zero. This type of parallel line connection allows an external source to supply the additional current and power at the rated voltage of the load. The connection can be made while the load is operating at full diesel power but the voltage of the external source must equal the required voltage of the load. Details of effecting smooth connect and disconnect transitions are familiar to those with knowledge of the art.
FIG. 3 illustrates a conventional series line connection system 300. In system 300, a diesel engine 310 drives an alternator 320 connected to a diode rectifier 330. If the voltage of an external source 315 is lower than the rated load voltage, source 315 can be connected in series with rectifier 330 to provide additional power while maintaining the required load voltage. In this type of series line connection, alternator 320 is operated at a reduced voltage so that the resulting load voltage remains at its rated valued. Rectifier 330 and external source 315 each carry the full load current but they contribute power proportional to their respective voltages. This mode is also referred to herein as Diesel Boost Operation since the voltage of external power source 315 is boosted by the diesel/alternator/rectifier combination to supply the load at its rated voltage and with higher power.
The use of a parallel line connection as in system 200 described with reference to FIG. 2 is limited to cases where the voltage of the external source is within the allowable operating voltage of the load.
The use of a series line connection as in system 300 described with reference to FIG. 3 has two serious problems. The first is that the alternator and rectifier must be oversized to handle the increased load current even though they operate at less than rated voltage while in series mode. For example, if the load power is doubled during Diesel Boost Operation and the external source supplies half the load voltage, then the alternator and rectifier must carry twice their rated current at half their rated voltage. While the alternator""s output power remains essentially the same as in system 100 described with reference to FIG. 1, the losses due to the high currents are prohibitive and this mode of operation is only possible for a very short time. It may be noted that prior systems typically use two windings and two rectifiers with series and parallel operating modes to avoid oversizing the components. But prior systems do not seek to provide more power by means of one (or more) additional sources connected in series.
The second problem is that there is considerable difficulty in switching from Diesel Operation (system 100) to Series Line Operation (system 300) without shutting off or disrupting uniform power to the load. The required load transfer must be rapid and not cause any abrupt change in load current or voltage. This requires the complicated steps of transferring the load current over a temporary commutation path, interrupting the rectifier (and alternator) output current, reducing the rectifier (and alternator) output voltage, connecting the external voltage source in series with the rectifier output, and then increasing the current to its former level to complete the commutation process. Only after this transition is complete can the current be increased to provide the desired higher load power. There is no economical method to known in prior systems to accomplish the required commutation process with a simple series line connection.
An approach to supply more current to the load may be to reconfigure the alternator windings into two parallel sets of windings (forming a dual winding alternator) and connecting the windings to two rectifiers as in prior systems. However, there are no known means heretofore to simultaneously maintain the load at its rated voltage (without drop off, perturbations, etc.) while connecting the two parallel connected rectifiers in series with the external source.
In summary, a parallel line connection such as system 200 described with reference to FIG. 2 will not work when the voltage of the available external source is less than the minimum rated voltage of the load. A series line connection such as system 300 described with reference to FIG. 3 is not practical because the size of the alternator and rectifier have to be increased to handle the higher currents and there is no feasible way to make the series connection with the external source while under power. A dual winding alternator with two parallel connected rectifiers will not work because the output voltage is too low for the load when the series connection is not present and there is no means to seamlessly effect the series connection to the external source.
Heretofore, there has been no means for resolving the foregoing problems.
The present invention relates to a system that overcomes the drawbacks mentioned above by providing a low-voltage/high-current source PL to supply additional power to a high voltage load without appreciably dropping the flow of power to the load or raising the voltage of the load while operating within the rating constraints of the first and second sources PD, PL such that the power transfer is seamless, i.e., transparent to the load. As such, no anomalies in power (current or voltage interruptions, spikes, oscillations, drop-off, perturbations, etc.) are sensed by the load.
In an exemplary embodiment, the invention doubles the current rating of a high-voltage low-current source PD, allowing it to operate indefinitely (i.e., extended periods of time) at increased currents required by a series connection with a low-voltage high-current source PL in order to operate a load at a higher power level.
The present invention further provides the second source PL to be seamlessly connected (and seamlessly disconnected) in series with the first source PD while the load is operating under full power.
The resulting circuit topology and control strategy overcomes the disadvantages of both the parallel and series line connections that previously rendered them unsuitable for this application. This invention can make it possible to connect a high-current source in series with an operating low-current source and increase the load current to its limit.